Carbon foam abrasives

ABSTRACT

The incorporation or blending of from about 1 to about 10% by volume of a “carbide precursor” powder, preferably on the order of &lt;100 microns in size, with a coal particulate starting material and the subsequent production of carbon foam in accordance with the method described herein, results in a carbon foam that exhibits significantly enhanced abrasive characteristics typical of those required in the polishing of, for example glass, in the manufacture of cathode ray tubes.

This application is a continuation of U.S. application Ser. No.10/068,074 filed Feb. 5, 2002, now U.S. Pat. No. 6,869,455, which is acontinuation-in-part of U.S. application Ser. No. 09/976,425 filed Oct.12, 2001, now U.S. Pat. No. 6,860,910.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract no.N00014-00-C-0062 awarded by Office of Naval Research.

FIELD OF THE INVENTION

The present invention relates to carbon foam materials and moreparticularly to coal-based carbon foams that include reaction bondedabrasive materials.

BACKGROUND OF THE INVENTION

There exists a continuing need for improved and enhanced abrasivematerials that exhibit high strength and excellent abrasion propertiesfor application in such areas as the polishing of glass for use incathode ray tubes and the like.

U.S. patent application Ser. No. 09/453,729 filed Dec. 2, 1999 andentitled, “Coal-Based Carbon Foams” describes a family of high strength,carbon foams having a density of preferably between about 0.1 g/cm³ andabout 0.8 g/cm³ produced by the controlled heating of coal particulatepreferably under a non-oxidizing atmosphere. The coal-based carbon foamsproduced in accordance with the method described in this application canbe carbonized and graphitized to yield very strong carbon foams that areextremely resistant to oxidation and ablation.

The graphitized carbon foams described in this application, exhibitcompressive strengths up to about 6000 psi and untreated demonstrateabrasive properties significantly better than those of the untreatedcoal-based carbon foams.

OBJECT OF THE INVENTION

It is an object of the present invention to improve the abrasiveproperties of the carbon foam materials of the aforementioned U.S.patent application Ser. No. 09/453,729 without adversely affecting anyof their other properties, particularly their strengths.

SUMMARY OF THE INVENTION

It has now been discovered that the incorporation of from about 1 toabout 10 volume percent of a carbide precursor such as titanium,silicon, tungsten etc. in a finely powdered form into the initial coalpowder starting material described in aforementioned U.S. patentapplication Ser. No. 09/453,729 results in the formation of the carbidesof these materials upon foaming, carbonization and graphitization. Suchcarbides are of course well known abrasives and their incorporation intothe already high strength, oxidation resistant and inherently ablationresistant carbon foams previously described results in significantlyimproved abrasive materials.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the heat treatment temperatures for thevarious phases of the production process used in the fabrication of theabrasive carbon foam of the present invention.

FIGS. 2-4 show comparative X-ray diffraction patterns for a variety ofcarbide precursor doped abrasive carbon foams in accordance with thepresent invention.

DETAILED DESCRIPTION

U.S. patent application Ser. No. 09/453,729 filed Dec. 2, 1999 andentitled, “Coal-Based Carbon Foams”, which is incorporated herein byreference in its entirety, describes a family of high strength, carbonfoams having a density of preferably between about 0.1 g/cm³ and about0.8 g/cm³ produced by the controlled heating of coal particulatepreferably under a non-oxidizing atmosphere. The coal-based carbon foamsproduced in accordance with the method described in this application arecarbonized and graphitized to yield very strong carbon foams that areextremely resistant to oxidation and ablation in their own right, buteven more so when produced in accordance with the method describedherein.

The process described in this prior application comprises: 1) heating acoal particulate of preferably small i.e., less than about ¼inchparticle size in a “mold” and under a non-oxidizing atmosphere at a heatup rate of from about 1 to about 20° C. per minute to a temperature ofbetween about 300 and about 700° C.; 2) soaking at a temperature ofbetween about 300 and 700° C. for from about 10 minutes up to about 12hours to form a green foam; and 3) controllably cooling the green foamto a temperature below about 100° C. According to the method describedin the aforementioned application, the green foam is subsequentlypreferably carbonized by the application of known techniques, forexample, soaking at a temperature of between about 800° C. and about1200° C. for a period of from about 1 to about 3 hours. Although this isthe preferred temperature range for carbonization, carbonization canactually occur at temperatures between about 600° C. and 1600° C.Graphitization, commonly involves heating the green foam either beforeor after carbonization at a heat-up rate of less than about 10° C. perminute, preferably from about 1° C. to about 5° C. per minute, to atemperature of between about 1700° C. and about 3000° C. in anatmosphere of helium or argon and soaking for a period of less thanabout one hour. Again, the inert gas may be supplied at a pressureranging from about 0 psi up to a few atmospheres.

The temperature ranges for the various thermal treatments of thematerials described in the aforementioned patent application and in thisapplication are shown graphically in FIG. 1. The foams thus producedexhibit a significant resistance to oxidation and ablation and can serveas abrasives in their own right.

It has now been discovered that the incorporation or blending of fromabout 1 to about 10% by volume of a “carbide precursor” powder,preferably on the order of <100 microns in size, with the coalparticulate starting material and the subsequent production of carbonfoam in accordance with the method described herein, results in a carbonfoam that exhibits significantly enhanced abrasive characteristicstypical of those required in the polishing of, for example glass, in themanufacture of cathode ray tubes.

“Carbide precursors” of the type useful in accordance with the presentinvention include but are not limited to such materials as silicon thatforms silicon carbide, tungsten that forms tungsten carbide and titaniumthat forms titanium carbide during calcination and graphitization asdescribed herein. However, any material capable of reacting with carbonduring the calcination and graphitization operations as described hereinto form an abrasive carbide that is “reaction bonded” to the carbon foamskeleton are suitable candidates for application in the abrasive carbonfoams described herein. “Reaction bonded” carbides comprise thosecarbides that are reactively bonded to the foam structure or skeletonduring the foam, calcination and graphitization processes, as opposed tosimilar carbide materials that might simply be added as a blend with thestarting material coal and remain in their unreacted state as “free”carbides (i.e. unbonded) in the final carbon foam product.

The starting material coal may include bitumen, anthracite, or evenlignite, or blends of these coals that exhibit a “free swell index” asdetermined by ASTM D720 of between about 3.5 and about 5.0, but arepreferably bituminous, agglomerating coals that have been comminuted toan appropriate particle size, preferably to a fine powder below about−60 to −80 mesh and exhibit “free swell indices” between about 3.75 andabout 4.5.

It is critical to the successful practice of the present invention thatthe coal starting material exhibit the previously specified free swellindex of between about 3.5 and about 5.0 and preferably between about3.75 and about 4.5. Selection of starting materials within theseparameters was determined by evaluating a large number of coalscharacterized as ranging from high to low volatiles. In general, it hasbeen found that bituminous coals exhibiting free swell indexes withinthe previously specified ranges provided the best foam products in theform of the lowest calcined foam densities and the highest calcined foamspecific strengths (compressive strength/density). Such bituminous coalsthat also possess the foregoing set of properties, high volatile content(35% to 45% by weight), large plastic range (at least about 50° C.),etc. and are thus characterized as high volatile bituminous coals, formthe preferred starting materials of the process of the presentinvention. Coals having free swell indices below the specified preferredranges may not agglomerate properly leaving a powder mass or sinter, butnot swell or foam, while coals exhibiting free swell indices above thesepreferred ranges may heave upon foaming and collapsed upon themselvesleaving a dense compact.

Additionally, according to further highly preferred embodiments of thepresent invention the coal starting materials of the present inventionpossess all or at least some of the following characteristics: 1) avolatile matter content (dry, ash-free basis) of between about 35 andabout 35% as defined by ASTM D3175, “Test Method for Volatile Matter inthe Analysis of Coal and Coke”; 2) a fixed carbon (dry basis) betweenabout 50 and about 60% as defined by ASTM D3172, “Practice for ProximateAnalysis of Coal and Coke”; 3) a Gieseler initial softening temperatureof between about 380° C. and about 400° C. as determined by ASTM D2639,“Test Method for Plastic Properties of Coal by the Constant-TorqueGieseler Plastometer”; 4) a plastic temperature range above about 50° C.as determined by ASTM D2639; 5) a maximum fluidity of at least 300 ddpm(dial divisions per minute) and preferably greater than about 2000 ddpmas determined by ASTM D2639; 6) expansion greater than about 20% andpreferably greater than about 100% as determined by Arnu Dilatation; 7)vitrinite reflectance in the range of from about 0.80 to about 0.95 asdetermined by ASTM D2798, “Test Method for Microscopical Determinationof the Reflectance of Vitrinite in Polished Specimens of Coal”; 8) lessthan about 30% inert maceral material such as semifusinite, micrinit,fusinite, and mineral matter as determined by ASTM D2798; and 9) nosignificant oxidation of the coal (0.0 vol % moderate or severeoxidation) as determined by ASTM D 2798 and non-maceral analysis.

The low softening point (380-400° C.) is important so that the materialsoftens and is plastic before volatalization and coking occur. The largeplastic working range or “plastic range” is important in that it allowsthe coal to flow plastically while losing mass due to volatalization andcoking. Vitrinite reflectance, fixed carbon content and volatile mattercontent are important in classifying these coal starting materials as“high-volatile” bituminous coals that provide optimum results in theprocess of the present invention and thus, carbon foam materials thatexhibit an optimum combination of properties when prepared in accordancewith the process described and claimed herein. The presence of oxidationtends to hinder fluidity and consequently, foam formation.

Thus according to various preferred embodiments of the presentinvention, a coal particulate starting material characterized as ahigh-volatile bituminous coal containing from about 35% to about 45% byweight (dry, ash-free basis) volatile matter, as defined by ASTM D3175,is a basic requirement for obtaining optimum results in the form ofoptimum carbon foaming in accordance with the process of the presentinvention. The various parameters derived from the Gieseler plasticityevaluations form the second highly important set of characteristics ofthe starting material coal if optimum results are to be obtained. Thus,a softening point in the range of from about 380° C. and about 400° C.,a plastic range of at least about 50° C. and preferably between about 75and 100° C., and a maximum fluidity of at least several hundred andpreferably greater than 2000 ddpm(dial divisions per minute) are highlyimportant to the successful optimized practice of the present invention.Accordingly, in order to obtain the carbon foams exhibiting the superiorproperties described herein, it is important that the coal startingmaterial be a high volatile bituminous coal having a softening point asjust described and a plastic range on the order of above about 50° C.all with the indicated Gieseler fluidity values described. Exhibition ofArnu dilatation values greater than about 20% and preferably above about100% when combined with the foregoing characteristics provideindications of a highly preferred high volatile bituminous coal startingmaterial.

The carbon foam abrasives described herein are semi-crystalline or moreaccurately turbostratically-ordered and largely isotropic i.e.,demonstrating physical properties that are approximately equal in alldirections. The abrasive carbon foams of the present invention typicallyexhibit pore sizes on the order of less than 300μ, although pore sizesof up to 500μ are possible within the operating parameters of theprocess described. The thermal conductivities of the cellular coal-basedproducts are generally less than about 1.0 W/m/°K. Typically, theabrasive carbon foams of the present invention demonstrate compressivestrengths on the order of from about 2000 to about 6000 psi at densitiesof from about 0.3 to about 0.5 g/cm³ and between about 2200 and about300 psi at densities between about 0.3 g/cm³ and about 0.4 g/cm³.

The method of producing the abrasive carbon foams of the presentinvention comprises initially: 1) heating a coal particulate ofpreferably small, i.e. less than about ¼ inch particle size, blendedwith from about 1 to about 10 volume percent of a “carbide precursor”powder in a “mold” and under an inert or non-oxidizing atmosphere at aheat up rate of from about 1 to about 20° C. perminute to a temperatureof between about 300 and about 600° C.; 2) soaking at a temperature ofbetween about 300 and 600° C. for from about 10 minutes up to about 12hours to form a “green foam”; 3) controllably cooling the “green foam”to a temperature below about 100° C.; carbonizing the green foam in aninert or non-oxidizing atmosphere to produce a carbonized foam; andgraphitizing. The inert or non-oxidizing atmosphere may be provided bythe introduction of inert or non-oxidizing gas into the “mold” at apressure of from about 0 psi, i.e., free flowing gas, up to about 500psi. The inert gas used may be any of the commonly used inert ornon-oxidizing gases such as nitrogen, helium, argon, CO₂, etc.

Blending of the coal particulate and the “carbide precursor” can becarried out in any of a number of conventional fashions. For example,dry blending of the coal particulate and the “carbide precursor” in aball mill works entirely satisfactorily. Other blending methods include,wet or solvent jar milling and multiple cycle co-pulverization using aHolmes disc pulverizer or the like. So long as a satisfactorily uniformand intimate mixture of the components is obtained, the particularmethod of blending is not of critical importance.

It is generally not desirable that the reaction chamber be vented orleak during this heating and soaking operation. The pressure of thechamber and the increasing volatile content therein tends to retardfurther volatilization while the cellular product sinters at theindicated elevated temperatures. If the furnace chamber is vented orleaks during soaking, an insufficient amount of volatile matter may bepresent to permit inter-particle sintering of the coal particles thusresulting in the formation of a sintered powder as opposed to thedesired cellular product. Thus, according to a preferred embodiment ofthe present process, venting or leakage of non-oxidizing gas andgenerated volatiles is inhibited consistent with the production of anacceptable cellular product.

Additional more conventional blowing agents may be added to theparticulate prior to expansion to enhance or otherwise modify thepore-forming operation.

The term “mold”, as used herein is meant to define any mechanism forproviding controlled dimensional forming of the expanding coal or carbonor containing the foaming operation. Thus, any chamber into which thecoal particulate and carbide precursor blend is deposited prior to orduring heating and which, upon the foam precursor attaining theappropriate expansion temperature, contains the expanding carbon to somepredetermined configuration such as: a flat sheet; a curved sheet; ashaped object; a building block; a rod; tube or any other desired solidshape can be considered a “mold” for purposes of the instant invention.The term “mold” as used herein, is also meant to include any container,even an open topped container that “contains” the expanding mixture solong as such a device is contained in a pressurizable vessel that willpermit controlled foaming as described herein. Clearly, a container thatresults in the production of some particular near net or net shape isparticularly preferred.

As will be apparent to the skilled artisan familiar with pressurized gasrelease reactions, as the pressure in the reaction vessel, in this casethe mold increases, from 0 psi to 500 psi, as imposed by the inert ornon-oxidizing gas, the reaction time will increase and the density ofthe produced porous coal will increase as the size of the “bubbles” orpores produced in the expanded carbon decreases. Similarly, a low soaktemperature at, for example about 400° C. will result in a larger poreor bubble size and consequently a less dense expanded coal than would beachieved with a soak temperature of about 600° C. Further, the heat-uprate will also affect pore size, a faster heat-up rate resulting in asmaller pore size and consequently a denser expanded coal product than aslow heat-up rate. These phenomenon are, of course, due to the kineticsof the volatile release reactions which are affected, as just described,by the ambient pressure and temperature and the rate at which thattemperature is achieved. These process variables can be used to customproduce the expanded coals of the present invention in a wide variety ofcontrolled densities, strengths etc.

Cooling of the “green foam” after soaking is not particularly criticalexcept as it may result in cracking of thereof as the result of thedevelopment of undesirable thermal stresses. Cooling rates less than 10°C./min to a temperature of about 100° C. are typically used to preventcracking due to thermal shock. Somewhat higher, but carefullycontrolled, cooling rates may however, be used to obtain a “sealed skin”on the open cell structure of the product as described below. The rateof cooling below 100° C. is in no way critical.

After expanding the carbon material as just described, the “green foam”is an open celled material. Several techniques have been developed for“sealing” the surface of the open celled structure to improve itsadhesive capabilities for further fabrication and assembly of a numberof parts. For example, a layer of a commercially available graphiticadhesive (for example an epoxy-graphite adhesive) can be coated onto thesurface and cured at elevated temperature or allowed to cure at roomtemperature to provide an adherent skin. Alternatively, the expansionoperation can be modified by cooling the “green foam” rapidly, e.g., ata rate of 10° C./min or faster after expansion. It has been discoveredthat this process modification results in the formation of a more denseskin on the “green foam” which presents a closed pore surface to theoutside of thereof. At these cooling rates, care must be exercised toavoid cracking.

After expanding, the “green foam” is readily machineable, sawable andotherwise readily fabricated using conventional fabrication techniques.

A variety of additives and structural reinforcers may be added to thecarbon materials of the present invention either before or afterexpansion to enhance specific mechanical properties such as fracturestrain, fracture toughness and impact resistance. For example,particles, whiskers, fibers, plates, etc. of appropriate carbonaceous orceramic composition can be incorporated into the abrasive foam toenhance its mechanical properties.

The abrasive carbon foams of the present invention can additionally beimpregnated with; for example, petroleum pitch, epoxy resins or otherpolymers using a vacuum assisted resin transfer type of process. Theincorporation of such additives provides load transfer advantagessimilar to those demonstrated in carbon composite materials. In effect a3-D composite is produced that demonstrates enhanced impact resistanceand load transfer properties.

The cooling step in the expansion process results in some relativelyminimal shrinkage on the order of less than about 5% and generally inthe range of from about 2% to about 3%. This shrinkage must be accountedfor in the production of near net shape or final products of specificdimensions and is readily determinable through trial and error with theparticular carbon starting material being used. The shrinkage may befurther minimized by the addition of some inert solid material such ascoke particles, ceramic particles, ground waste from the coal expansionprocess etc. as is common practice in ceramic fabrication.

According to the method of the present invention, subsequent to theproduction of the “green foam” as just described, the “green foam” issubjected to carbonization and graphitization within the controlledconditions described below to obtain the abrasive foam of the presentinvention.

Carbonization, sometimes referred to as calcining, is conventionallyperformed by heating the green foam under an appropriate inert gas at aheat-up rate of less than about 5° C. per minute to a temperature ofbetween about 600° C. and about 1600° C. and preferably between about800° C. and about 1200° C. and soaking for from about 1 hour to aboutthree or more hours. Appropriate inert gases are those described abovethat are tolerant of these high temperatures. The inert atmosphere issupplied at a pressure of from about 0 psi up to a few atmospheres. Thecarbonization/calcination process serves to remove all of the non-carbonelements present in the green foam such as sulfur, oxygen, hydrogen,etc.

Graphitization, commonly involves heating the carbon foam either beforeor after carbonization at heat-up rate of less than about 10° C. perminute, preferably from about 1° C. to about 5° C. per minute, to atemperature of between about 1700° C. and about 3000° C. in anatmosphere of helium or argon and soaking for a period of less thanabout one hour. Again, the inert gas may be supplied at a pressureranging from about 0 psi up to a few atmospheres. According to apreferred embodiment of the process described herein, the abrasive foamsof the present invention are produced by sequentially carbonizing andthen graphitizing the green foam as described above.

As will be apparent to the skilled artisan, graphitization according tothe method described herein inherently results in carbonization in thecourse of the graphitization process. Thus, although carbonization isrecited and may indeed be performed as a descrete operation, it is infact a portion of the graphitization procedure being achieved as the“green foam” passes through the carbonization thermal regimen on its waytoward graphitization at a higher temperature.

Typically, the abrasive carbon foams described herein exhibit thefollowing additional properties at a density of between about 0.3 g/cm³and about 0.4 g/cm³: tensile strength, 300-1000 psi; shear strength 300psi; and impact resistance 0.3-0.4 ft-lbs/in².

The following examples will serve to better illustrate the successsfulpractice of the invention.

EXAMPLE

Three samples of high volatile bituminous Upper Elkhorn (Pike County,Ky.) coal containing about 30% by weight of volatile matter were dopedwith 3% by volume of tungsten, titanium and silicon respectively, arefoamed at a temperature between about 450 and 500° C. under an inertatmosphere of helium as 500 psi using a 2° C. per minute heat up rateand a 2 hour residence at temperature to form the green foam. The greenfoam was then carbonized in an electric resistance furnace at atemperature of 1050° C. using a 0.5° C. heat up rate and a residencetime of two hours. The carbonized abrasive foam was then graphitized ata temperature of 2200° C. using the procedures described abouve.

As shown in FIGS. 2-4, X-ray diffraction analysis of each of the samplesshowed the presence of the anticipated metallic carbide when compared toX-ray diffraction patterns of the undoped graphite matrix, the puremetallic carbide and silicon carbide as a control in two of the cases.It is thus apparent that doping by blending of the carbide precursorpowder as described herein produced a graphitized carbon foam matrixthat incorporated the expected metallic carbide.

Evaluations of these materials showed that their abrasive properties aresignificantly better than those of the undoped graphitized carbon foam.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

1. An abrasive carbon foam comprising: an open-cell carbon foam; and acarbide reactively bonded to the open-cell carbon foam; wherein theabrasive carbon foam is impregnated with a polymer.
 2. The abrasivecarbon foam of claim 1, wherein a surface of the abrasive carbon foam issealed.
 3. The abrasive carbon foam of claim 1, wherein the carbide issilicon carbide.
 4. The abrasive carbon foam of claim 1, wherein thecarbide is tungsten carbide.
 5. The abrasive carbon foam of claim 1,wherein the carbide is titanium carbide.
 6. The abrasive carbon foam ofclaim 1, wherein the polymer is an epoxy resin.
 7. An abrasive carbonfoam comprising: an open-cell carbon foam; and a carbide reactivelybonded to the open-cell carbon foam; wherein the abrasive carbon foam isimpregnated with a petroleum pitch.
 8. The abrasive carbon foam of claim7, wherein a surface of the abrasive carbon foam is sealed.
 9. Theabrasive carbon foam of claim 7, wherein the carbide is silicon carbide.10. The abrasive carbon foam of claim 7, wherein the carbide is tungstencarbide.
 11. The abrasive carbon foam of claim 7, wherein the carbide istitanium carbide.
 12. An abrasive carbon foam, comprising: asemi-crystalline porous coal-based structure having a density rangingfrom about 0.1 to about 0.8 g/cm³, wherein the semi-crystalline porouscoal-based structure includes a metallic carbide reaction bonded to theporous coal-base structure for improving abrasive character of theabrasive carbon foam, and wherein the relative amount of reaction bondedcarbide to the semi-crystalline porous coal-based structure is an amountranging from about 1 to about 10% by volume.
 13. The abrasive carbonfoam of claim 12, wherein the density ranges from about 0.2 to about 0.5g/cm³.
 14. The abrasive carbon foam of claim 12, wherein the densityranges from about 0.3 to about 0.4 g/cm³.
 15. The abrasive carbon foamof claim 12, wherein the carbide is tungsten carbide.
 16. The abrasivecarbon foam of claim 12, wherein the carbide is silicon carbide.
 17. Theabrasive carbon foam of claim 12, wherein the carbide is titaniumcarbide.